Possibly the two joints corresponding to the two pins are not tenoned, but there were quite a few tongues in the structure overall. Thanks for the mortice/mortise clarification! I never knew that!
As a bit of trivia, these are recycled chestnut logs (well, maybe not THESE, to be fair -- many of the logs used in the structures at the site are, and I ASSUME that these are as well). Some bloke apparently preserved a bunch of structures in western virgina by taking them apart and warehousing the logs. This, basically, means there's a warehouse full of chestnut logs all broadaxed to rough shape morticed here and there, some with tenons as well. So there are mortises cut into these things more or less at random, with tenons from other members occasionally put in as needed. These logs have been used at least once before, and possibly several times. It makes things quite attractive. Of course it's also rather sad to know that the species is pretty much wiped out.